The cost estimate and the final legislative language were expected last week, but both have subsequently been delayed. The healthcare bill could be unveiled as soon as Monday evening or Tuesday.

Another complicating factor is the health of Sen. Robert Byrd (D-W.Va.), who has missed more than 130 roll call votes this year.

Reid needs every member of his party because Republicans have indicated they will vote en masse against the Democratic legislation.

Senior Democrats believe their party members will fall into line when the time comes. But public statements by two centrists in particular — Sens. Blanche Lincoln (D-Ark.) and Ben Nelson (D-Neb.) — have given Reid and other leaders reason for concern.

The Iowa senator laid out an ambitious schedule for the final weeks before the end of the year and said Reid is committed to working through every weekend in December if that’s what it takes to pass the bill before lawmakers break for the holiday.

“We’re going to be going long days — I’ve already talked to Leader Reid about this — long nights, weekends — constantly, from then until right before Christmas, when I think we’ll have the votes, hopefully, to pass the bill,” he said.

But Democrats remain divided over key elements yet to be resolved: whether to create a government-run insurance program, how to pay for expanding insurance coverage and how strongly to prohibit federal dollars from paying for abortion services.

First Reid must win over his centrists to begin the debate. Pressure on the party holdouts is expected to intensify from Senate Democratic leaders, the White House and liberal activists, but the centrists have their own political situations to consider.

Lincoln is the most at risk. She is up for reelection next year in a red state amid what is shaping up to be a bad environment for Democrats. Lincoln, who has misgivings about the public option, has also complained about being asked to support the procedural motion without first seeing the final bill.

Nelson has withheld a commitment to vote on the motion to proceed and criticized the bill on a number of issues. He has left the door open to support a Republican filibuster at any stage in the legislative process.

Landrieu continues to be targeted despite telling the Huffington Post last month, “I’m not right now inclined to support any filibuster. … I’m not going to be joining people that don’t want progress.” But liberals remain dubious, especially after Landrieu told reporters last week that she had not decided what she would do, according to press reports. A Landrieu spokeswoman did not respond to a request for comment.

Bayh received a torrent of liberal criticism for several days last month when he seemed to suggest he would not necessarily vote in favor of the motion to proceed on the healthcare bill. He quickly clarified that on the first vote, at least, he would vote with his party.

The one wildcard is Sen. Olympia Snowe (Maine), who voted to approve the Finance Committee measure. But Snowe has since indicated Reid lost her vote by including a public option in the bill that the full Senate will consider.

Getting past the motion to proceed, of course, will be an easier task than those that lie ahead. Centrists like Lieberman and liberals like Sen. Roland Burris (D-Ill.) have indicated that while they will vote with Reid on the first procedural motion, they could oppose or even filibuster the bill itself.